xcrypt.3: Warn folks not to use these functions

xcrypt.3: Warn folks not to use these functions

There is not an acceptable reason to use these functions ever in
new code.  For example, just observe the implementation of the
KDF:

	/*
	 * Turn password into DES key
	 */
	void
	passwd2des_internal (char *pw, char *key)
	{
	  int i;

	  memset (key, 0, 8);
	  for (i = 0; *pw && i < 8; ++i)
	    key[i] ^= *pw++ << 1;

	  des_setparity (key);
	}

This kind of nonsense isn't okay in the year 2017. Therefore, we
enlighten our poor users.

[Note from mtk: I think Jason knows that of which he talks.]

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>